Diffraction grating

ABSTRACT

Provided is a diffraction grating having a surface with a groove-and-ridge structure formed with a preset periodic distance, the groove-and-ridge structure including a plurality of grooves, wherein a concave structure and/or a convex structure extending across at least one of the grooves and having a level difference from the bottom surface of the groove is formed on the surface. In the diffraction grating according to the present invention, the diffracting grooves in the groove-and-ridge structure formed with the preset periodic distance are divided into a plurality of sections. Therefore, even if oil or water contained in air-born dust, the sebum or perspiration of a user, or a similar liquid substance is adhered to the surface of the diffraction grating, the oil or the like will not spread over an area other than the section to which it has adhered. Thus, a decrease in the diffraction efficiency is prevented.

TECHNICAL FIELD

The present invention relates to a diffraction grating. Morespecifically, it relates to a diffraction grating having a specificallydevised structure for preventing a decrease in the diffractionefficiency due to contamination of the diffracting surface.

BACKGROUND ART

A diffraction grating is an optical element for separating white light,i.e. the mixture of various wavelengths of light, into wavelengthcomponents. Such an element is used in a spectroscope or similar opticalinstruments. Diffraction gratings are roughly divided into two types:reflection grating, which separates incident light into wavelengthcomponents by reflecting the light on its surface, and transmissiongrating, which allows incident light to penetrate into and pass throughits body to produce a spectrum of light separated into wavelengthcomponents. In any of the reflection and transmission types, the gratinghas a surface with a groove-and-ridge structure cyclically formed with apreset periodic distance corresponding to the wavelength range to beused (for example, see Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: JP 2004-198461 A

SUMMARY OF INVENTION Technical Problem

Normally, the surface of a diffraction grating which is set in aspectroscope is exposed to the air. Therefore, various substances (e.g.oil or water) contained in the air-born dust adhere to the surface ofthe diffraction grating. Furthermore, in the process of setting thediffraction grating in the spectroscope, the sebum or perspiration fromthe user's fingers may adhere to the surface of the diffraction grating.Such a liquid substance 4 containing oil and/or the like (see FIG. 1A,where the upper drawing is a plan view and the lower drawing is a frontview) which has adhered to the surface of the diffraction grating 21 andentered the grooves 22 on the surface of the diffraction gratinggradually spreads and fills the grooves 22 (see FIG. 1B), which lowersthe diffraction efficiency of the grating. FIG. 2 is a photograph of thesurface of a diffraction grating with diffracting grooves ruled on it inthe vertical direction, which shows an oil or similar contaminant widelyspread along the diffracting grooves.

The problem to be solved by the present invention is to provide adiffraction grating whose diffraction efficiency will not besignificantly lowered even if oil or water contained in air-born dust,the sebum or perspiration of a user, or similar liquid substance isadhered to the surface of the diffraction grating on which thegroove-and-ridge structure is formed.

Solution to Problem

The present invention aimed at solving the previously described problemis a diffraction grating having a surface with a groove-and-ridgestructure formed with a preset periodic distance, the groove-and-ridgestructure including a plurality of grooves, and the diffraction gratingbeing characterized in that a concave structure and/or a convexstructure cutting across at least one of the grooves and having a leveldifference from the bottom of the groove is formed on the surface.

In the diffraction grating according to the present invention, among thegrooves formed on the surface of the diffraction grating with a presetperiodic distance, each groove which is cut across the concave or convexstructure having a level difference from the bottom surface of thegroove is divided into a plurality of sections by the concave or convexstructure. Therefore, even if an oil or similar liquid substance isadhered to the surface, the adhered oil or the like cannot spread fromone section to another. Specifically, in the case where the concavestructure is provided, the oil or the like spreading from the section towhich it has adhered enters the concave structure and cannot reach othersections. In the case where the convex structure is provided, the oil orthe like spreading from the section to which it has adhered is blockedby the convex structure and cannot reach other sections. Therefore, thesituation in which the adhered oil or the like spreads over the entiregroove and significantly lowers the diffraction efficiency will notoccur as in the conventional case.

Users often hold a diffraction grating with their fingers at the edgesof the grating. Accordingly, in order to prevent the thereby adheredsebum or perspiration of the user from spreading into the central regionof the surface of the diffraction grating, the concave structure or theconvex structure should preferably be provided in a marginal region ofthe surface of the diffraction grating. The marginal region meansoutside the central region on the surface of the diffraction gratingonto which a beam of light is irradiated when the grating is used in aspectroscope. Given the fact that the skin of fingertips holding aspectroscope protrudes inwards by approximately 1 mm, the concavestructure or the convex structure only needs to be located atapproximately 2 mm inside from the edge of the surface of thespectroscope (or up to approximately 5 mm with some allowance). Thisdesign makes no influence on the cyclic ridge-and-groove structure inthe central region of the surface of the diffraction grating whichserves as the primary passage for light. Therefore, no decrease in thediffraction efficiency occurs due to the provision of the concave orconvex structure.

Advantages Effects of the Invention

In the diffraction grating according to the present invention, theridge-and-groove structure formed on the surface of the diffractiongrating with a preset periodic distance is divided into a plurality ofsections. Therefore, even if oil or water contained in air-born dust,the sebum or perspiration of a user, or a similar liquid substance isadhered to the surface of the diffraction grating on which thegroove-and-ridge structure is formed, the oil or the like will notspread over an area other than the section to which it has adhered.Therefore, the diffraction efficiency will not significantly decrease asin the conventional case.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B illustrate how an oil or similar contaminant spreads ona surface of a conventional diffraction grating.

FIG. 2 is a photograph showing a surface of a diffraction grating with acontaminant spread on the diffracting surface.

FIG. 3 illustrates one embodiment of the diffraction grating accordingto the present invention.

FIGS. 4A and 4B illustrate how an oil or similar contaminant spreads ona surface of the diffraction grating of the aforementioned embodiment.

FIG. 5 illustrates another embodiment of the diffraction gratingaccording to the present invention.

FIG. 6 illustrates still another embodiment of the diffraction gratingaccording to the present invention.

DESCRIPTION OF EMBODIMENTS

One embodiment of the diffraction grating according to the presentinvention is hereinafter described with reference to the drawings.

FIG. 3 shows a plan view (upper drawing) and a front view (lowerdrawing) of the diffraction grating 1 of the present embodiment. Thediffraction grating 1 of the present embodiment is a laminar gratinghaving a surface with cyclic rectangular grooves 2 forming theridge-and-groove structure. Five convex structures 3 orthogonal to therectangular grooves 2 are provided at regular intervals. In thediffraction grating 1 of the present embodiment, the level of the top ofthe convex structures 3 is slightly higher than that of the ridges ofthe ridge-and-groove structure cyclically formed on the surface of thediffraction grating. Since the convex structures 3 only need to divideeach rectangular groove 2 into a plurality of sections, the top of theconvex structures 3 may be lower than that of the ridges of the cyclicgroove-and-ridge structure as long as it is higher than the bottomsurface of the rectangular groove 2.

The spreading of an oil or similar liquid substance 4 which has adheredto the diffracting surface of the diffraction grating 1 of the presentembodiment and entered the rectangular grooves 2 is hereinafterdescribed with reference to FIGS. 4A and 4B. FIG. 4A shows thediffraction grating 1 of the present embodiment with the oil or similarliquid substance 4 adhered to the same locations as in FIG. 1A. Asalready explained, in the case of the conventional diffraction grating11, the oil or similar liquid substance 4 which has entered thediffraction grooves 22 gradually spreads over wide areas along thediffraction grooves 22 (see FIG. 1B) and significantly lowers thediffraction efficiency. By contrast, in the case of the diffractiongrating 1 of the present embodiment, since the rectangular grooves 2 aredivided into a plurality of sections by the convex structures 3, thespreading of the oil or similar liquid substance 4 is limited within thesections to which the liquid substance has adhered, as shown in FIG. 4,and there is no influence on the neighboring sections. Therefore, thediffraction efficiency will not be significantly lowered as in the caseof the conventional diffraction grating 11.

It is also possible to divide the rectangular grooves 2 into a pluralityof sections by providing concave structures deeper than the rectangulargrooves 2. In this case, when the adhered oil or similar liquidsubstance 4 reaches an end of one section, the substance enters theconcave structure and cannot spread over the neighboring section. Thus,the decrease in the diffraction efficiency can be prevented whichever ofthe convex and concave structures is used to divide the rectangulargrooves 2 into multiple sections.

At the locations where the convex or concave structures are provided,the cyclic pattern of the groove-and-ridge structure is lost and thedesired diffraction of light does not occur. Therefore, the intensity ofthe diffracted light will be too low if the convex or concave structuresare provided in excessive numbers or with an excessive size.Accordingly, it is preferable to control the number and size of theconvex or concave structures so that the total area of those structureswill be no greater than 5% of the area of the surface of the diffractiongrating on which the cyclic groove-and-ridge structure is formed.

Normally, when incident light is separated into wavelengths by means ofa diffraction grating, an intense light is irradiated onto the centralregion of the surface of the diffraction grating. Meanwhile, theadhesion of the sebum or perspiration of the user is likely to occurwhen the user holds the diffraction grating with its end. Accordingly,it is preferable to provide the convex or concave structures in themarginal region of the surface of the diffraction grating and not in itscentral region. Given the fact that the skin of fingertips holding aspectroscope protrudes inwards by approximately 1 mm, the spreading ofthe sebum or perspiration of the user into the central region can beadequately prevented by providing the concave or convex structures atapproximately 5 mm inside from the edge of the surface of thediffraction grating. If it is certain that users will carefully handlethe diffraction grating, the convex or concave structures may be locatedat approximately 1 mm from the edge. By such a design, the oil orsimilar liquid substance which adheres to the diffraction grating whenthe user holds the grating can be prevented from spreading into thecentral region, without causing a decrease in the diffraction efficiencydue to the creation of the convex or concave structures.

FIG. 5 shows one example; the upper drawing is a plan view of thediffraction grating 11 while the lower drawing is a front view (left)and a C-C′ sectional view (right) of the same grating. The diffractiongrating of the present embodiment is a 5 cm×5 cm laminar grating havingtwo concave structures 5 orthogonal to the rectangular grooves 2provided at a distance of 5 mm from each end of the surface of thediffraction grating. The concave structures 5 are deeper than the bottomsurface of the rectangular grooves 12. As already explained, if theconcave structures 5 are provided, any liquid substance which adheres tothe ends of the surface of the diffraction grating of the diffractiongrating while being held by a user enters the concave structures 5 andis thereby prevented from spreading into the central region of thesurface of the diffraction grating, so that the diffraction efficiencywill not decrease.

In any of the previous embodiments, the convex structures 3 or theconcave structures 5 formed orthogonally to the rectangular grooves.However, those structures do not always need to be orthogonal to thegrooves. FIG. 6 shows another possible example, in which the rectangulargrooves 2 are divided into a plurality of sections by providing convexstructures 6 each of which obliquely extends with respect to therectangular grooves 2. It is also possible to give the convex or concavestructures a curved form (e.g. a circular form).

The configuration of the present invention can be applied to anydiffraction grating having a surface on which a groove-and-ridgestructure is formed with a preset periodic distance. For example, thepresent invention can also be applied to a diffraction grating havingsaw-tooth grooves or sinusoidal grooves, other than a diffractiongrating having a cyclic groove-and-ridge structure formed by rectangulargrooves as in the previous embodiments. In those cases, the saw-tooth orsinusoidal grooves should be divided into a plurality of sections byproviding a convex structure higher than the bottom of the grooves or aconcave structure lower than the bottom of the grooves.

The present invention can be applied to both reflection grating andtransmission grating.

The present invention is not limited to a diffraction grating whosesurface having the groove-and-ridge structure formed with a presetperiodic distance is flat; it can also be applied to a diffractiongrating having a concave or convex surface.

REFERENCE SIGNS LIST

-   1, 11, 21 . . . Diffraction Grating-   2, 12, 22 . . . Groove (Rectangular Groove)-   3, 6 . . . Convex Structure-   4, 14 . . . Liquid Substance-   5 . . . Concave Structure

1. A diffraction grating having a surface with a groove-and-ridgestructure formed with a preset periodic distance, the groove-and-ridgestructure including a plurality of grooves, and the diffraction gratingbeing characterized in that a concave structure and/or a convexstructure cutting across at least one of the grooves and having a leveldifference from a bottom of the groove is formed on the surface.
 2. Thediffraction grating according to claim 1, wherein the concave structureand/or the convex structure is provided in a marginal region of thesurface of the diffraction grating.
 3. The diffraction grating accordingto claim 1, wherein a total area of the concave structure and/or theconvex structure is no greater than 5% of an area of the surface of thediffraction grating on which the cyclic groove-and-ridge structure isformed.
 4. The diffraction grating according to claim 2, wherein a totalarea of the concave structure and/or the convex structure is no greaterthan 5% of an area of the surface of the diffraction grating on whichthe cyclic groove-and-ridge structure is formed.